The overall aim of this study is to examine the postnatal development of fever production. This project was motivated by the common, but little studied, clinical experience that the newly born of many species, including human, when, afflicted with infectious disease are generally unable to educe the characteristic rise in body temperature, although other available evidence indicates that thermo- regulatory ability is extant from birth. The method of study is by analysis of the ontogenic changes in various functional components that are prototypic of the pyrogenic response to endotoxin of adult animals, in particular, the increased thermogenesis and the release of leucocytic pyrogen.